1. Field of the Invention
The present invention relates to a switching power source in which the power factor is increased by producing a stable DC power source from an AC power source and operating so that an input voltage and an input current of a chopper circuit are approximately in phase and have similar waveforms.
2. Description of the Related Art
Among the control methods for switching power sources of the above kind are a critical-mode control method in which control is made so that a switching element is turned on when a coil current becomes zero and a continuous-current-mode control method in which control is made so that a coil current does not become zero. In general, the critical-mode control method is advantageous for the purpose of noise reduction, as switching occurs when the coil current becomes zero, and hence switching noise is low. However, its application to uses with a heavy load is difficult because large current ripples occur and hence much stress is imposed on a coil and a diode.
On the other hand, the continuous-current-mode control method can be applied to heavy-load uses by virtue of small current ripples and low stress on a coil and a diode, though the switching noise is larger than in the critical-mode control method. Generally available control ICs operate only in the critical mode or the continuous current mode. Therefore, it is necessary to use different control ICs depending on the load. This increases the development cost because, for example, the design of a power source system needs to be modified.
FIG. 11 shows an exemplary switching power source of the continuous-current-mode control type that is disclosed, for example, in JP-A-04-168975. In FIG. 11, reference numeral 1 denotes an AC power source; 2, a rectification circuit; 3 and 6, capacitors; 4, an inductor; 5, a diode; 7, a switching element such as a MOSFET; 8, a voltage error amplifier; 9, a multiplier; 10, a comparator; 11, a monostable multivibrator; 12, a current detection resistor; and 13, a drive circuit.
An output voltage of the AC power source 1 is full-wave-rectified by the rectification circuit 2, which is a diode bridge. High frequency noise is removed from a rectified voltage by the capacitor 3 (input voltage Vin), and a current is supplied to the capacitor 6 via the inductor 4 and the diode 5, whereby a smoothed DC voltage Vout is obtained. The switching element 7 is connected to the connecting point of the inductor 4 and the diode 5, and when turned on shunts the current flowing from the inductor 4 to the diode 5.
The voltage error amplifier 8 amplifies an error of the output voltage Vout with respect to a reference voltage Vref and gives an amplified error voltage Verr to the multiplier 9. The multiplier 9 multiplies the error voltage Verr and the input voltage Vin together, whereby a threshold value signal Vth is generated which is in phase with and similar, in waveform, to the input voltage Vin and is proportional, in amplitude, to the error voltage Verr. On the other hand, the current flowing through the inductor 4 is converted by the current detection resistor 12 to a current detection signal Vi, which is compared with the threshold value signal Vth by the comparator 10. An output of the comparator 10 is input to the trigger input terminal of the monostable multivibrator 11. The output of the monostable multivibrator 11 is kept at a low level for a prescribed time from input of a trigger signal and is thereafter changed to a high level. The output of the monostable multivibrator 11 is input to the drive circuit 13. The drive circuit 13 turns on the switching element 7 when its input turns to the high level, and turns off the switching element 7 when its input turns to the low level.
With the above configuration, when the switching element 7 is turned on, the current flowing through the inductor 4 increases and the current detection signal Vi rises. When the current detection signal Vi exceeds the threshold value signal Vth, the output of the comparator 10 turns to the high level. A trigger signal is input to the monostable multivibrator 11 and its output turns to the low level, whereby the switching element 7 is turned off via the drive circuit 13. The current coming from the inductor 4 decreases gradually. Since the low-level period of the monostable multivibrator 11 is set so that the current flowing through the inductor 4 does not become zero, the output of the monostable multivibrator 11 turns to the high level and the switching element 7 is turned on via the drive circuit 13 when the current has decreased to some extent.
FIGS. 12A and 12B illustrate the above operation in which control is made so that peaks of the current detection signal Vi (which corresponds to the current flowing through the inductor 4) coincide with the threshold value signal Vth which is in phase with and similar, in waveform, to the input voltage Vin. Since the on time is variable and the off time is fixed, the switching frequency varies and the frequency of generated noise also varies. As a result, the noise spectrum is spread and noise reduction is enabled. FIG. 12A illustrates a relationship between the threshold value signal Vth and the current detection signal Vi, and FIG. 12B shows an on/off waveform of the switching element 7.
To increase the power factor, it is necessary to make an input current in phase with and similar, in waveform, to an input voltage. To this end, it is necessary to vary the on duty cycle broadly from 0% to a value close to 100%. A voltage developing across the inductor when the input voltage is of a 100 V system is different from that when the input voltage is of a 200 V system. In addition, the voltage across the inductor always varies even in each cycle of an AC input voltage. Therefore, the time constant (di/dt) of a current flowing through the inductor varies greatly. As a result, the current variation in a fixed time varies greatly depending on the value and the phase of an input voltage and the load state. Therefore, if the off time is fixed as in the above conventional example, the current variation becomes excessive or insufficient. This means a problem that the power factor cannot be increased beyond a certain limit. In the critical-mode control method, the switching element needs to be turned on when the current becomes zero. The critical-mode control is difficult in the case where the off time is fixed as in the above conventional example.
A method known as a measure against the above problems is disclosed in JP-A-2007-143383. This reference proposes a power factor increasing method in which a second threshold value signal Vth2 is provided which is proportional to a first threshold value signal Vth1. A switching element is turned off when a voltage corresponding to a current flowing through an inductor reaches the first threshold value signal Vth1, and is turned on when the voltage becomes lower than the second threshold value signal Vth2. According to this method, neither the on time nor the off time is fixed and the switching element can be turned on and off automatically with optimum timing in accordance with the input voltage state and the load state. The power factor can thus be increased.
FIGS. 13A and 13B show an exemplary control method according to JP-A-2007-143383. As shown in FIGS. 13A and 13B, where the second threshold value signal Vth2 varies in proportion to the first threshold value signal Vth1, the input voltage Vin which is a full-wave-rectified voltage of an AC power source voltage varies with time and accordingly the first threshold value signal Vth1 varies between a reference potential and a peak potential. When the first threshold value signal Vth1 is around the reference potential, the second threshold value signal Vth2 is also small and the difference between the first threshold value signal Vth1 and the second threshold value signal Vth2 is small. The amplitude of the coil current becomes small and the switching frequency becomes very high, resulting in a problem that noise and switching loss are increased. FIG. 13A illustrates a relationship among the first and second threshold value signals Vth1 and Vth2 and the current detection signal Vi. FIG. 13B shows an on/off waveform of the switching element.
Where the ratio of the second threshold value signal Vth2 to the first threshold value signal Vth1 is fixed at a prescribed value, the difference between the first threshold value signal Vth1 and the second threshold value signal Vth2 is small (see FIG. 14B) when the average current flowing through the inductor is small because of an input voltage and a load condition. In this case, the switching frequency is high, resulting in a problem that noise and switching loss are increased. FIG. 14A shows a case that the difference between the first threshold value signal Vth1 and the second threshold value signal Vth2 is relatively large.